Method and apparatus for direct writing of semiconductor die using microcolumn array

ABSTRACT

In electron beam lithography, a lithography system uses multiple microcolumns in an array to increase throughput for direct writing of semiconductor wafers. The mismatch between the microcolumn array and the semiconductor die periodicity is resolved by using only one microcolumn to scan each individual die. This is accomplished by assuring that the stage carrying the semiconductor wafer moves a total distance in each of the X and Y directions which is greater than the pitch between adjacent die. Hence each die is scanned by only a single microcolumn although at possibly different times during the total stage motion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to lithography and more specifically to chargedparticle (e.g. electron beam) lithography using microcolumns to directwrite images onto a wafer.

2. Description of the Prior Art

Most lithography now used for fabricating semiconductor devices useslight which passes through a mask, where the mask carries the image tobe transferred to a photosensitive resist layer on a substrate. A secondtype of well known lithography is electron beam lithography. Instead ofusing light, this uses an electron beam (but no mask) to form an imageon a substrate. The substrate is coated with a layer of resist sensitiveto the incident electron beam. The resist in either case is thendeveloped and the exposed areas then either remain or are removed,defining a pattern on a surface of the wafer. Subsequent steps etch awaythe exposed portions of the wafer surface to define semiconductorfeatures.

To date, electron beam lithography has been used mostly to fabricate themasks rather than the semiconductor wafers. The masks themselves arethen used in the photolithography, as disclosed above. However, it isalso known to use electron beam lithography to direct write featuresonto a semiconductor wafer. Typically this is only used for lowthroughput systems where a small number of semiconductor chips areneeded, since the direct write approach is relatively slow. Of course ithas the advantage of eliminating the masks and also providing very smallfeature sizes (better resolution) due to the nature of the electronbeams compared to the much longer wavelength light used inphotolithography. Thus to date practical applications of high resolutionelectron beam lithography are typically limited to mask making andmanufacturing of highly specialized integrated circuits, due to the lowthroughput and high equipment cost for electron beam lithography.

However, since the general trend in semiconductor fabrication is toreduce minimum feature size and there is an expectation of a minimumfeature size below 100 nanometers in the next 10 years, photolithographyis becoming increasingly expensive and may not offer sufficientresolving power. Minimum feature size is typically the minimum width ofa portion of a transistor as defined the lithography process which inturn defines the overall size of the transistor and hence the number oftransistors or other semiconductor devices which may be provided on asingle integrated circuit.

While integrated circuits are typically called chips, at the wafer leveland prior to packaging they are often referred to as die. Thatterminology is used herein to refer to a single semiconductor substratewhich will later become a semiconductor integrated circuit. A typicalsemiconductor wafer contains many (for instance hundreds or thousandsof) such die arranged in a grid.

Therefore while it has been widely recognized that electron beamlithography has possibilities for direct writing of mass productionsemiconductor wafers, so far this has not been commercially feasible.

Also known in the electron beam lithography field is a technology calledmicrocolumns. A typical electron beam lithography machine has a singlesource of electrons, an associated accelerator (electrostatic) devicefor accelerating the electrons, and a set of elements which aretypically coaxial electro-magnets for focusing the beam onto thesubstrate. However, it is known (see e.g. U.S. Pat. Nos. 5,155,412 and5,122,663 to IBM and "Electron-beam microcolumns for lithography andrelated applications", incorporated herein by reference) to provide anarray of so called microcolumns wherein each individual microcolumn is acomplete electron beam column including an electron beam source, anaccelerator or extractor electrode, a deflector electrode for scanningthe beam, and a electrostatic lens for focusing the beam. Suchmicrocolumns have a typical diameter of approximately 1 to 2centimeters. A two dimensional array of such microcolumns has beenproposed for lithography.

Also, while the disclosure herein is mostly directed to electron beamlithography, electrons of course are just one type of charge particles.Also known are ion beam sources which instead of emitting electrons emitother charged particles. Unlike an electron beam system, instead ofmerely requiring a source of electric current to the source, a source ofatoms (a gas) must be provided. Hence such devices are generally slowerin writing speed and more complicated than electron beam devices and sofar have not been used commercially for lithography.

In any case, it would be desirable to provide a method of increasingthroughput (production rates) in semiconductor fabrication usingelectron or ion beam technology so as to allow direct writing ofsemiconductor wafers for mass production of high volume integratedcircuits. So far, this has not been feasible.

SUMMARY

While the use of an array of electron beam microcolumns for directwriting of semiconductor devices has been proposed, the presentinventors have discovered certain problems therein. The general idea, asis well known, is to use multiple electron beam microcolumns to increasethroughput as compared to an electron beam machine having only a singlecolumn. Since each microcolumn has the full complement of beam forming,beam deflection, and beam blanking capabilities of a conventional singlecolumn, well-established scanning electron beam writing techniques canbe used in such a microcolumn machine. Typically, patterns are writtenover relatively narrow stripe of less or equal 100 μm in width andbutted (connected) using a continuously moving stage which carries thesemiconductor wafer. The position of the stage is controlled by laserinterferometry as is conventional.

This type of writing using an array of microcolumns can use the wellknown MEBES raster scan writing approach currently used for making maskswith a single electron beam column. See e.g. U.S. Pat. Nos. 4,818,885 toIBM and 4,668,083, and 3,900,737 to Bell Labs, incorporated herein byreference. Thus a basic system architecture in accordance with thepresent invention may include the basic data path and many otheradvanced techniques such as multi-path and multipixel techniquesdeveloped for MEBES. See U.S. Pat. Nos. 5,621,216 to IBM; 5,393,987 toEtec System Inc., and 5,103,101 to Etec Systems, Inc., incorporatedherein by reference. Also, other well known electron beam lithographyapproaches such as using a gray scale or shaped beam can also be used;see e.g. U.S. Pat. Nos. 5,213,916 to IBM; 5,334,467 to IBM; 4,568,861 toIBM; 4,423,305 to IBM incorporated herein by reference.

However, the present inventors have determined that there is asignificant problem using a microcolumn array to direct writesemiconductor die on a semiconductor wafer. This is the problem ofmismatch between the microcolumn array pitch and the die layout pitch.

The mismatch problem is that the microcolumn array has a pitch (centerto center distance) between adjacent microcolumns in both dimensions ofthe array of e.g. 1 centimeter, where this is the typical diameter ofthe individual microcolumns including the housing of each microcolumn.Of course it is desirable in any one of such electron beam lithographymachines to have the individual microcolumns rigidly fixed in relationto each other so as to maintain optimum alignment and accuracy. However,the individual die to be written by such a machine typically vary insize depending on the complexity of the circuitry on the die and otherfactors. Hence the fixed arrangement of the microcolumn array and theneed to accommodate varying sizes of the die are problematic.

One obvious solution to this is to provide a mechanically adjustablearray of microcolumns, whereby each microcolumn may be moved in both theX and Y directions (which is the plane defined by the substrate to bewritten) relative to one another. While this is theoretically possible,it is probably undesirable because of the very precise calibration andaccuracy requirements.

However in accordance with the invention, the present inventors havedeveloped a writing method and apparatus that solve this problem withoutrequiring any particular mechanical or adjustment features, but insteaduse a particular writing approach. In accordance with this approach,suppose the array of microcolumns has a 1 centimeter pitch (center tocenter distance) while the die are on a 1.5 centimeter pitch. Byassuring that the stage carrying the wafer moves a total distance(travel) greater than the die pitch in both directions (X and Y) ratherthan only moving a distance equal to the die pitch, then each die willalways be covered completely in terms of being scanned (written to) byonly a single microcolumn.

Of course, each particular die may be so covered at different timesduring the total stage motion of scanning the entire wafer. Thereforesometimes not every microcolumn will be used to scan a particular wafer,depending on the difference between the die pitch and the microcolumnpitch. Hence in accordance with the invention, there is some redundancyof microcolumns and a certain increase in printing time. However, thisredundancy can be used to provide additional reliability in themicrocolumn array since it will always be possible to select differentsubsets of the microcolumn array to write particular die sizes, perhapsat some additional throughput decrease.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows diagrammatically the technical problem addressed by thisinvention and the solution.

FIG. 2 shows alignment in accordance with the invention.

FIG. 3 shows diagrammatically data processing and control of themicrocolumns and stage in accordance with this invention.

DETAILED DESCRIPTION

The method in accordance with this invention and the problem it solvesare both illustrated in FIG. 1 which shows diagrammatically a wafersubstrate 10 (in plan view) on which there are a number of individualdie shown by the solid black lines in an array. Of course a typicalsubstrate includes hundreds or thousands of die rather than only nine asillustrated here; FIG. 1 is merely for purposes of conceptualexplanation. It is to be understood that the die are merely particularareas on the substrate and are defined by the semiconductor featuresformed thereon. The substrate is later conventionally scribed (sawnapart) along the dark lines.

In this example, the individual die are squares each measuring 1.5centimeter by 1.5 centimeter in respectively the X and Y axisdirections, thus having a 1.5 centimeter pitch P. In contrast, themicrocolumn array 16 (which is shown here only in terms of the areas tobe imaged by each microcolumn), includes a number of microcolumns(dotted lines) which are arrayed so that they are placed on a 1centimeter pitch C. Each individual microcolumn area shown by the dottedlines measures only 1 centimeter by 1 centimeter. Hence there isobviously a mismatch between the periodicity between the microcolumnarray 16 and the die array on wafer 10.

As described above, in accordance with this invention an array ofmicrocolumns 16 can be scanned over a different sized array of die onwafer 10 while assuring that each individual die is scanned by only onemicrocolumn. This is done by simply assuring that the stage carrying thewafer 10 moves a total distance (travel) of D in this case, for instance2 centimeters, which is a distance greater than the die pitch P, in eachof the X and Y directions for each stage movement rather than only 1.5centimeters. Thus each and every die area will always be coveredcompletely by only one microcolumn for scanning. Of course, possiblyportions of each individual die will be scanned at different timesduring the total stage motion.

Thus in this case the microcolumns have an exemplary pitch of C of 1centimeter, the die pitch P is 1.5 centimeters, and the total stagemovement D in each direction for each movement of the stage is 2centimeters. Of course these dimensions are merely exemplary. The keyfactor is that the total range of motion D of the stage in each of the Xand Y directions is greater than the microcolumn pitch in thatdirection. Therefore in accordance with the invention this achieves theresult that each individual die area is written by only a singlemicrocolumn, rather than by multiple microcolumns as would be thesituation if the stage range of movement was otherwise. This method hasthe disadvantage that not every microcolumn will be used in such ascanning approach, depending on the difference between the die size andthe microcolumn array periodicity. Also of course there is somereduction in throughput due to the relatively inefficient use of themicrocolumns.

By using this method to accommodate differences between the microcolumnarray and the die periodicity, a fixed microcolumn array size can beused. Hence one lithography machine ("tool") with a fixed array ofmicrocolumns can be used to pattern wafers having a variety of diesizes.

However, before the actual patterning process, an accurate calibrationof the beam position in each column and the scan width (or stripe width)is performed. This is accomplished by the aid of a very accuratecalibration grid 18 as shown in FIG. 2 with periodicity matching that ofthe fixed column-array 16. The grid 18 is placed at the same height asthe wafer 10, and can be incorporated into the stage (not shown) thatcarries the wafer 10 or be inserted as needed into the system. The grid18 is fabricated with suitable topography and/or material to allow anadequate secondary or reflected electron signal to be generated. Thecalibration process involves each column 16 scanning the grid 18, andthe signals so derived allow the beam position and scan width of eachcolumn 16 to be properly adjusted.

Referring back to FIG. 1, in the usual case, an identical pattern is tobe imaged onto each individual die. In this case the writing method issuch that each field (a portion of the die pattern) for each die iswritten in exactly the same manner except for the timing of the datasupplied to the individual die areas of FIG. 1. Hence FIG. 3 shows asimplified graphic representation of the data path for a microcolumnarray direct write tool which writes one die per microcolumn. It is tobe understood that the actual lithography tool is basically a multiplemicrocolumn electron beam lithography tool of the type known in the art;one element changed here is the control program for providing the data(which defines the pattern to be written) to the individual microcolumnsand for controlling the stage movement.

Hence in one embodiment the invention is partially embodied in computersoftware which is part of the program used to control the lithographytool. The lithography tool may be of the MEBES type except having anarray of microcolumns. FIG. 3 shows the pattern data which isconventionally for a single die (chip) provided to the rasterizer 24which is either conventional electronic hardware or a computer programrunning on the computer or microprocessor (not shown) which controls thelithography tool. The rasterizing 24 process itself is essentiallyconventional of the type used in the well known MEBES technology. Therasterizer 24 then feeds the rasterized pattern data to each of avariety of independently controlled variable time delay buffers B1through BN. There is one such buffer for each microcolumn in themicrocolumn array 16; the microcolumns are therefore designated in FIG.3 M1 through MN. The actual microcolumns are typically arranged in a twodimensional array but this is of course not limiting.

The variable time delay buffers B1 through BN are such that the timedelay provided by each buffer is set by the system independent of therasterizer 24. The rasterizer 24 thereby drives all of the microcolumnsM1 through MN and it rasterizes only a single pattern which is thepattern desired to be written on each individual die. Further the datastream provided to each microcolumn M1 through MN is independent of themicrocolumn alignment, again with the exception of the timingdifferences as provided by the buffers B1 through BN.

Thus this data path is designed on an individual die basis rather than afull wafer basis. This is a significant simplification compared to whatis now used for microcolumn systems where "stitching" betweenmicrocolumn patterns is inherent. (Stitching here is the process wherebymore than one column writes a single die.) In accordance with thisinvention, no such stitching is needed because each individual die ispatterned by only a single microcolumn.

As shown in FIG. 3, each microcolumn M1 through MN receives exactly thesame data, but the buffers B1 through BN allow timing variations for thewriting action by each microcolumn. This delay compensates for thedifference in periodicities illustrated in FIG. 1 when one particularmicrocolumn is over a particular portion of its die, the adjacentmicrocolumn will be over a different portion of its die. Hence the delaybuffers B1 through BN correct for this. The electron beams E1, . . . ,EN are emitted from microcolumns M1, . . . , MN respectively andblanked, deflected, etc. by conventional elements in each microcolumn.Each microcolumn also includes a conventional electron magnetic lens L1,. . . , LN in one version. The electron beams are incident on wafer 10which is supported on movable stage 28. Movement of stage 28 in theX,Y,Z axes is controlled as described above by e.g. computer (orcontroller) 30 which is coupled to rasterizer 24. It is to be understoodthat FIG. 3 is a high level block diagram. However given thisdescription, one of ordinary skill in the art can write a suitablecomputer program or design a suitable hardware rasterizer to carry outthe method described herein.

This description is illustrative but not limiting; further modificationswill be apparent to one skilled in the art in light of this disclosureand are intended to fall within the scope of the appended claims.

We claim:
 1. A method of imaging a pattern on a substrate, the patterndefining a plurality of die on the substrate, there being a pitch Pbetween adjacent die, using an array of charged particle beam columns,comprising:moving the substrate in two perpendicular directions whileblanking the charged particle beams on and off, thereby scanning thepattern onto the substrate; wherein a total distance D the substrate ismoved in each of the two directions for each movement in either of thetwo directions is greater than P, so that only one column images each ofthe die in the pattern.
 2. The method of claim 1, further comprising,for each column, individually forming the beam, deflecting the beam, andblanking the beam on and off.
 3. The method of claim 1, furthercomprising operating each of the particle beams at a plurality ofintensities.
 4. The method of claim 1, further comprising shaping thebeams.
 5. The method of claim 1, wherein the moving comprises scanningthe pattern in a multipass or multipixel format.
 6. The method of claim1, wherein the array of columns has a pitch C between adjacentmicrocolumns, and C is less than P.
 7. The method of claim 1, wherein awidth of the beam from each column at the substrate surface is less than100 μm.
 8. The method of claim 6, wherein D=2 C.
 9. The method of claim1, further comprising:providing a grid having a periodicity matchingthat of the array of columns; and aligning the array of columns relativeto the substrate using the grid.
 10. The method of claim 1, whereinduring the scanning of the pattern onto the substrate, at least one ofthe columns is not used.
 11. The method of claim 2, wherein each columnis an electron beam microcolumn.
 12. The method of claim 1, furthercomprising variably delaying provision of data defining the pattern toeach of the columns.
 13. The lithography apparatus for imaging a patternonto a substrate, the pattern defining a plurality of die on thesubstrate, there being a pitch P between adjacent die, comprising:anarray of charged particle beam columns arranged to each direct a chargedparticle beam onto the substrate to define the pattern, each columnincluding an individually controlled beam blanker; a support for thesubstrate, the support being movable in two perpendicular directions forscanning the pattern onto the substrate; and a control coupled to thestage for controlling the stage movement, wherein the control moves thestage a total distance D in each of the two directions for each movementin either of the two directions, where D is greater than P, so that onlyone column images each of the die in the pattern.
 14. The apparatus ofclaim 13, each column further including an individually controlled beamdeflector.
 15. The apparatus of claim 13, each column further includinga gray scale beam intensity control.
 16. The apparatus of claim 13, eachcolumn including a beam shaper.
 17. The apparatus of claim 13, whereinthe control moves the stage and controls the beams so as to scan thepattern in a multipass or multipixel format.
 18. The apparatus of claim13, wherein the array of columns has a pitch C between adjacent columnsand C is less than P.
 19. The apparatus of claim 13, wherein a width ofthe beam from each column at the substrate surface is less than 100 μm.20. The apparatus of claim 18, wherein D=2 C.
 21. The apparatus of claim13, further comprising:a grid having the same periodicity as the arrayof columns and located on the support; and means for aligning the arrayof columns relative to the substrate using the grid.
 22. The apparatusof claim 13, wherein during the scanning of the pattern at least one ofthe columns is not used.
 23. The apparatus of claim 13, wherein each ofthe columns is an electron beam microcolumn.
 24. The apparatus of claim13, further comprising a variable delay buffer coupled to provide datadefining the pattern to each column.